C Questions And Answers – 2018A2

There are many commonly asked questions regarding C programming language. Below are some collected such question-answer examples. The questions are usually related with Turbo C IDE in windows or GCC under Linux environment [not always].

For more such examples, click C_Q&A label.

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What are memory models?

 

Ans:

The compiler uses a memory model to determine how much memory is allocated to the program. The PC divides memory into blocks called segments of size 64 KB. Usually, program uses one segment for code and a second segment for data. A memory model defines the number of segments the compiler can use for each. It is important to know which memory model can be used for a program. If we use wrong memory model, the program might not have enough memory to execute. The problem can be solved using larger memory model. However, larger the memory model, slower is your program execution. So we must choose the smallest memory model that satisfies our program needs. Most of the compilers support memory models like tiny, small, medium, compact, large and huge.

 

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How do I know how many elements an array can hold?

 

Ans:

The amount of memory an array can consume depends on the data type of an array. In DOS environment, the amount of memory an array can consume depends on the current memory model (i.e. Tiny, Small, Large, Huge, etc.). In general an array cannot consume more than 64 kb. Consider following program, which shows the maximum number of elements an array of type int, float and char can have in case of Small memory model.

 

main( )

{

int i[32767] ;

float f[16383] ;

char s[65535] ;

}

 

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How do I write code that reads data at memory location specified by segment and

offset?

 

Ans:

Use peekb( ) function. This function returns byte(s) read from specific segment and offset locations in memory. The following program illustrates use of this function. In this program from VDU memory we have read characters and its attributes of the first row. The information stored in file is then further read and displayed using peek( ) function.

 

#include <stdio.h>

#include <dos.h>

 

main( )

{

char far *scr = 0xB8000000 ;

FILE *fp ;

int offset ;

char ch ;

if ( ( fp = fopen ( "scr.dat", "wb" ) ) == NULL )

{

printf ( "\nUnable to open file" ) ;

exit( ) ;

}

// reads and writes to file

for ( offset = 0 ; offset < 160 ; offset++ )

fprintf ( fp, "%c", peekb ( scr, offset ) ) ;

fclose ( fp ) ;

if ( ( fp = fopen ( "scr.dat", "rb" ) ) == NULL )

{

printf ( "\nUnable to open file" ) ;

exit( ) ;

}

// reads and writes to file

for ( offset = 0 ; offset < 160 ; offset++ )

{

fscanf ( fp, "%c", &ch ) ;

printf ( "%c", ch ) ;

}

fclose ( fp ) ;

}

 

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…till next post, bye-bye & take care.